1. Field of the Invention
The invention is in the field of control circuits for vehicle drive systems and is particularly applicable for vehicles having a differential mechanism dividing torque between two output shafts.
2. Description of the Prior Art
Various mechanisms have been devised in the automobile and trucking industry to control excessive slippage between the driving wheels of a vehicle. Such devices usually serve to equalize the rotational speed of two or more output shafts which are driven by a main drive or input shaft. The driven shafts may practically be referred to as output drive shafts since they are used to drive vehicle wheels either directly or through some intermediate mechanical linkage. Some differential in speed between these shafts is necessary to permit different rotational speeds of the driving wheels as the vehicle negotiates a turn, encounters bumps or holes in the roadway, or traverses rough terrain. Most typically, the output drive shafts are coupled by means of a differential to a main drive shaft or propeller shaft, and the differential provides the mechanism for dividing torque evenly between the output drive shafts and allowing for different rotational speeds in the output drive shafts. In the trucking industry, it is also advantageous to provide multi-axle tandem drive assemblies utilizing an inter-axle differential coupling the main propeller shaft from the engine to the differentials on each of the two rear driving axles hereafter referred to as the forward rear and rear rear drive axles.
Under normal operating conditions, when the vehicle is traveling on good roads and under dry whether conditions, excessive slip between output drive shafts is usually not encountered and no corrective actions are required. However, during adverse weather conditions the vehicle may be traveling through mud or ice and an exceptional amount of slippage may occur as when one of the wheels loses traction and begins to spin excessively, hereafter referred to as a "slip condition". It has therefore been advantageous to provide lockout mechanisms or other control devices to eliminate the excessive difference in rotational speeds of the differential output shafts.
Mechanical lockout mechanisms for coupling the main drive shaft to an output shaft of a differential have been utilized in the trucking industry and examples are shown in U.S. Pat. Nos. 3,264,901 and 3,390,593. Mechanical locking devices have also been utilized on inter-axle differentials of tandem drive roadway vehicles as illustrated in U.S. Pat. No. 2,870,853.
A ratio sensitive electronic control for limited slip differentials is disclosed in U.S. Pat. No. 3,138,970.
An exemplary teaching of an electromechanical system utilizing selective brake control to limit the speed differential between a pair of wheels of a vehicle is shown in U.S. Pat. No. 3,706,351.
Generally speaking, in a differential mechanism, by locking any two shafts of the group of three shafts consisting of the input drive shaft and the two output drive shafts one locks all three shafts and eliminates the "differential" function. The terms "lock-out," "lock condition" or "locked condition" generally refer to the condition wherein the differential mechanism coupling the main drive shaft to the two output shafts is rendered inoperable with the result that both output shafts rotate at the same speed, and torque delivered from the engine is provided to both output shafts as required by the external resistance each output shaft is subject to.
Lock-out may typically be achieved manually by the vehicle driver upon sensing a slip condition or may be achieved under automatic control as for example, in U.S. Pat. No. 3,138,970 mentioned above. Slip control may also be provided by means other than locking out a vehicle differential, and the brake control system of U.S. Pat. No. 3,706,351 constitutes an alternate approach to the problem. Electronic circuitry may thus be provided to control means for eliminating or at least decreasing the amount of slip to within acceptable limits.
Typically, the electronic control is responsive to sensed input speed signals and provides continuous monitoring and control. Such continuous monitoring systems are susceptible to repeated cycling of the control apparatus inasmuch as output shaft speeds tend to become synchronous almost immediately after lock-out thereby destroying the error signal before the vehicle is actually out of its slip condition. The electronic control continues to monitor the output shaft speeds and if indeed the vehicle is not out of its initial slip condition upon release of the locking device the error signal will be regenerated and the control locking device reapplied. Oscillations may typically occur within the drive system at a rather high frequency of between 1-3 Hz. Such oscillations may adversely affect the vehicle by repeatedly stressing the drive train components and may disturb the vehicle operator, particularly if the control system continues to recycle and the vehicle fails to traverse that portion of the road presenting the "slip" condition.